Turbochargers are a type of forced induction system. Turbochargers deliver air, at greater density than would be possible in a normally aspirated configuration. The greater air density allows more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. A smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, will reduce the mass of the engine and can reduce the aerodynamic frontal area of the vehicle.
With reference to FIG. 1, turbochargers use the exhaust flow from the engine exhaust manifold to drive a turbine wheel 10. The energy extracted by the turbine wheel is translated to a rotating motion which then drives a compressor wheel 20. The compressor wheel draws air into the turbocharger, compresses the air, and delivers it to the intake side of the engine. The rotating assembly is supported by a bearing system. Some bearing systems consist of sleeve type hydrodynamic bearings and some consist of rolling element type bearings.
As the mass flow of exhaust to the turbocharger changes, the rotational speed changes (from 80,000 RPM for large turbochargers, to 250,000 RPM for smaller turbochargers). Some of the parameters affecting the time for the rotating assembly to change from one equilibrium condition to another equilibrium condition are, for example: the inertia of the rotating assembly, the friction losses in the bearing system, and the aerodynamic efficiency of the wheels.
Electrically assisted turbochargers can use power supplied by an external source or power generated directly by the engine. The challenges of fitting an electric motor into a turbocharger are not minor. Most electrically assisted systems have either a connection to the (relatively) cold compressor-end of the rotating assembly or are fitted between the wheels. For example, U.S. Pat. No. 6,845,617 teaches an electric motor fitted to the compressor-end of the turbocharger outboard of the bearing system.
In the example depicted in FIGS. 1 and 2, an electric motor is disposed between the journal bearings in a split turbocharger bearing housing. The bearing housing is split into an upper portion 89 with a flange 91 and a lower portion 90 with a flange 92. When the two flanges (91, 92) are mechanically clamped together, the assembly functions as that of a unified turbocharger bearing housing. A laminated rotor 12 is mechanically mounted to the shaft 11 of the turbocharger such that it rotates about the axis 1 of the turbocharger with the shaft and wheels, becoming part of the rotating assembly of the turbocharger. A laminated stator 40, with power windings 42 providing the magnetic force to drive the aforementioned rotor 12, is mounted concentric with the rotor.
The surface finish and accuracy of the shaft surfaces, upon which the internal oil film for the journal bearings is generated, may have for example a surface finish of Rz4 coupled with a cylindricity requirement of 0.005 mm. The surface finish of the surfaces (24, 25), which support the journal bearings (49C, 49T) respectively, are sufficiently fine that they could not tolerate scratches or grooves generated by pressing the rotor 12 over these surfaces. To prevent damage to the journal bearing surfaces when the collars and rotor stack are assembled to the shaft 11, the diameters of the various portions of the shaft are stepped down towards the compressor-end of the shaft, which is the end of the shaft over which parts are assembled onto the shaft.
As depicted in FIG. 2, a ring boss 15 locates the piston ring 5 that provides a seal between the exhaust gases in the turbine stage and the oil and air within the bearing housing. The turbine-end journal bearing 49T is disposed about journal 25. The turbine-end electric motor collar 13T is secured to (i.e. pressed onto) diameter 26. Journal 25 is bound on one side by shoulder A, which is located between the ring boss 15 and journal 25. At the other end, journal 25 is defined by step B, which is located between journal 25 and diameter 26. Each transition to a different diameter along the shaft is referred to as a step. Each step is associated with a shoulder against which components may be located.
Rotor 12 is secured to the shaft along diameter 27. Step C marks the transition between diameter 26 and diameter 27. Compressor-end collar 13C is also secured to diameter 27. Compressor-end journal bearing 49C is disposed about journal 24. The transition between diameter 27 and journal 24 is marked by step D. Step S marks the transition between journal 24 and the stub shaft 16. The axial constraint, in the direction of the electric motor rotor, is provided by the clamping load of the compressor nut 17 on the compressor wheel 20, flinger 53, and thrust washer 52, against shoulder S.
While the above described multiple diameters provide assembly protection for the very accurate and fine surface finishes of the various portions of the shaft, the inside diameters of the compressor-end journal bearing 49C and motor collar 13C are smaller (and hence different) from those parts (49T, 13T) on the turbine side of the electric motor rotor 12. This difference means that the part number count per turbocharger increases and the potential for incorrect assembly of the journal bearings and motor collars exists. It also means that the journal bearings can run at different speeds since the speed defining features (inside diameter, ratio of inside diameter to outside diameter etc.) are different turbine-end to compressor-end. In this case, the compressor-end journal bearing will run at a lower speed than does the turbine-end journal bearing.
Accordingly, there is a need for a bearing system for use in an electrically assisted turbocharger that provides the desired bearing surface finishes while minimizing the complexity and part count associated with existing designs.